Emirates Team NZ's AC72 was sailing Wednesday in the Hauraki Gulf in a session that lasted for around six hours in 15-20kt winds.

Already the team has burned through 20% of its allowable sailing days, and by the end of next week will have been launched and sailing for two months.

Only one of those days has been reduced due to breakage - that happened on the second day when some broken ribs were noticed in her 40metre tall wing sail.

Of her rivals, only the Defender, Oracle Team USA has launched and sailed their AC72. In a sail that lasted just two hours before limping back to dock with a damaged starboard foil. Days after that incident the team issued a media statement saying that incident would keep the team off the water for several weeks.

Three of the five entered teams, Oracle Team USA, Emirates Team NZ and Artemis Racing (Sweden) are building two boats - the maximum allowed under the Protocol.

The first boats were allowed to be launched and sailing by July 1, 2012 and the second one may be launched after February 1, 2013.

While build times will vary for each team, the process is complex, and the lock-off point for the designs for the second boats, and the start of the build process is generally reckoned to be the end of September, aimed at a February 1, 2013 launch date, at which point they can begin two-boat testing for a period of 45 days over the next three months.

That puts both the other two-boat teams in the situation where they will likely be having to commence the build for their second boats without the benefit of significant performance and actual structural load data from the first. For the Defender the situation is not quite such an issue as they do not have to be ready for the Louis Vuitton Cup starting on July 4, 2013, but nevertheless cannot afford to lose significant sailing time.

For the other two teams, Luna Rossa and Team Korea, the time situation is not so serious, as they are intending to do just a single boat. Luna Rossa are expected to launch in October 2012 and will start a tune up program out of Auckland. They are constrained by the same sailing day limitations, but are expected to make good use of the approaching Auckland summer. Oracle Team USA was due to train out of a New Zealand base at Marsden Cove, but canned that idea and will instead winter over in San Francisco with Artemis racing for company.

The boats can be altered as they are being built, with areas beefed up, during the build phase (prior to measurement), but it is not an optimal situation.

Artemis Racing looked set to have stolen a march on the other teams with the use of a 60ft trimaran to test their wing sail sailing out of Valencia, Spain, ahead of the July 1 launch date . But in May, the wingsail broke in two, and while the team has continued with the build of their first boat, they have not sailed. A re-jigged plan saw them make the decision to re-locate early to San Francisco, which may help them claw back some valuable time.

All teams are constrained by a rule limiting them to 30 days sailing before February 1, 2013, which effectively means one day per week from the original launch date.

However the effect of the rule, coupled with other limitations in the Protocol, which governs the racing, mean that there is a premium on time spent early in the cycle (before October) to maximise the data collection against which the design assumptions and structural engineering calculations can be checked.

Add into the mix the fact that these boats can sail complete clear of the water on hydrofoils - enabling them to sail at double the true windspeed, plus the fact that they are using wingsails, and the fact that these are the largest yachts to be supported using hydrofoils, then there is really no known knowledge base on which the designers and engineers can draw. That makes the early knowledge even more vital, as opposed to the former IACC monohulls against which there was several cycles of data and design was a matter of fine tuning rather than the coarse testing of the limits that is underway now in Auckland.

That lack of base knowledge was underlined by Oracle Team USA with the damage to the daggerboard just two hours into their maiden sail - a session which should have lasted six hours.

In a further twist, teams are limited to the construction of 10 daggerboards, meaning five matched pairs, which can be used across the two boats. Construction of a new board would certainly lower that available tally, maybe as low as eight if the structural issue had been replicated in the other undamaged board, which could not be fixed without a substantial rebuild or unplanned redundancy.

So far Emirates Team NZ have clocked up around 32 hours on the water. The next team has done just two. The Kiwis' testing seems to have been marked with a lack of breakdown, with the team twice doing a foiling flypast up the Auckland harbour at the end of a testing session, indicating that all was well aboard.

Head of the Emirates Team New Zealand design group, Nick Holroyd, is ultimately responsible for getting drawings to the builder.

'It puts massive premium on the sailing time,' Holroyd said. 'The pressure’s also high to get Boat 1 sailing consistently so that the sailing team can get the race training they need to do.'

'For the design team the aim is to get as much value out of sailing Boat 1, turn the data around very quickly and load it into Boat 2.'

While the Boat 2 design process is well advanced, 'we still have input into the big items – the hulls, the cross structures and the general layout.

'Because of the time constraints the reality is the team’s second boat will be version 1.5 rather than version 2. We would need six more months of work to make significant changes to the second boat.'

'The fact is that Emirates Team New Zealand’s boat is the only one on the water right now. Everyone’s finding these boats are very, very expensive, complicated and difficult to engineer.

'We would like to be nursing it but can’t; we have to push it to the upper teen wind range and conditions more likely to be experienced at San Francisco.

'It’s true that we are pushing the boat harder and faster than some of us are comfortable with. We have long nerve-wracking days on the water but they are producing a lot of valuable refinements to carry through to Boat 2.'

Winds on the Waitemata Harbour and Hauraki Gulf were in the 15-20kt range for the sixth day of testing.

The AC72 was expected to have reached speeds in excess of 40kts in these conditions, which are the strongest winds in which the AC72 has sailed to date. Air temperatures were some of the lowest of the year, and the wind chill factor on the very wet AC72 can only be imagined.